Geothermal brine and steam reservoirs exist in many areas of the world and are a valuable energy resource. Some steam reservoirs yield a superheated steam which, after treatment to remove contaminants, can be used to power a turbine connected to an electrical generator. More usually, the reservoir yields a geothermal brine which must be flashed to produce steam to power the turbine. After powering the turbine, the exhausted steam is condensed in either a direct contact condenser or a surface condenser (e.g., a shell-and-tube-condenser) to produce steam condensate. The steam condensate is then, in the vast majority of cases, used as liquid water make-up to a cooling tower which provides the working fluid (i.e., the cooling medium) for condensing steam in the condenser.
Cooling in the cooling tower is accomplished by evaporation, which produces, as a side effect, the concentration of salts, minerals, and chemicals in the non-evaporated water. If a direct contact condenser is used, as is the case with most geothermal power plants, the cycles of concentration in the cooling tower are normally not controlled. If a surface condenser is used, such as a tube-and-shell condenser, the cooling tower is usually operated to control the concentration of salts by maintaining the cycles of concentration within predetermined limits. This is accomplished by controlling the cooling tower blow down, i.e., controlling the rate at which liquid water is discharged from the cooling tower system.
Depending upon the location of the geothermal power plant, the percentage of total condensate produced in the turbine-condenser system which eventually is evaporated or discharged as cooling tower blowdown is between about 70 and 95%, leaving about 5 to 30% of excess condensate for disposal, e.g., discharge to the environment (i.e., by distribution upon a natural earth surface or by discharge to a water body, such as a lake, creek, river, or ocean). Ideally, where possible, the environmental discharge can also serve a beneficial purpose, e.g., agricultural irrigation. Alternatively, the excess condensate can be used for other beneficial purposes, e.g., industrial water. Alternatively again, the excess condensate can be re-injected into the geothermal resource formation, and in many cases this is desired to maintain the resource pressure and volume. However, in other cases, it is not desired, but becomes a necessity because the condensate contains one or more components in excess of applicable environmental discharge regulations. The cost of complying with such regulationsxe2x80x94i.e., the cost to construct one or more re-injection wells and the associated surface facilitiesxe2x80x94is quite high, on the order of $4 million.
One component dissolved in the excess condensate which may force the operator of a geothermal power plant to bear the cost of re-injection to comply with environmental regulations is boron. Geothermal brines and steam typically contain boron, and as a result the steam condensate obtained in the condenser contains boron.
Ironically, if the boron in the condensate could be controlled to low levels, its presence would actually be beneficial. Boron is one of sixteen important micro-nutrients needed for healthy crop growthxe2x80x94a factor favoring its presence in waters intended for agricultural purposes. On the other hand, boron in forms concentrated above the micronutrient level can inhibit starch formation and in yet higher concentrations prove toxic to plants. The Water Encyclopedia, Second Edition, by van der Leeden et al., Lewis Publishers, Inc. (1990), herein incorporated by reference in its entirety, specifies in Table 6-46 a 0.5 mg/l concentration as the xe2x80x9cthreshold levelxe2x80x9d below which the concentration xe2x80x9cshould be satisfactory for almost all crops and almost any arable soil.xe2x80x9d The xe2x80x9climiting concentration,xe2x80x9d xe2x80x9cat which the yield of high-value crops might be reduced drastically, or at which an irrigator might be forced to less valuable cropsxe2x80x9d is identified as 2.0 mg/l. These limits are consistent with the data in Table 6-49 of van der Leeden et al. wherein the xe2x80x9cpermissible limitsxe2x80x9d for boron are broken down by crop group. For those most tolerant to boron, e.g., onion, asparagus, and date palm, the permissible limits are between 2 and 3 mg/l. For semi-tolerant crops, such as sunflower, potato, wheat, corn and lima bean, the permissible limits are between 1.33 and 2 mg/l. And for the most sensitive crops, such as pecans, plum, apple, and most especially citrus and avocado, the permissible limits are from 0.67 to 1 mg/lxe2x80x94values which are very much in line with the proposed 0.6 to 1 mg/l limits proposed for Federal drinking water regulations. See xe2x80x9cAn Update of the Federal Drinking Water Regs,xe2x80x9d by Pontius, Journal AWWA, February, 1995, herein incorporated by reference in its entirety.
Due to the sensitivity of many crops to the presence of boron, the boron concentration in water used for agricultural purposes often must comply with local water quality regulations. Citrus are among the most sensitive receptors to boron and are adversely affected at a level of 0.75 mg/lxe2x80x94the limit for irrigation water in the Philippines. See in particular pages 4468 and 4473 of the NPCC Rules and Regulations, Official Gazette, Vol. 74, No. 23, pp. 4467-4476, (1978) herein incorporated by reference in its entirety. In addition, the Philippines, per the Ministry, Bureau and Office Administrative Orders and Regulations, Official Gazette, Vol. 78, No. 1, pp. 52-54, which document is herein incorporated by reference in its entirety, set a 2 mg/l limit on boron for discharge to certain inland waters. The U.S. effluent standard is also 2.0 mg/l, per the 1978 Effluent Standards of the National Pollution Control Commission, herein incorporated by reference in its entirety.
Where no local regulations exist for boron in irrigation water, it would stand to reason that any water supplied or sold for irrigation obviously should not contain boron in a concentration greater than the toxicity level for the plant under cultivation. For example, 2 mg/l boron in water is harmful for rice growth. Hence, to be on the safe side, the boron concentration for waters supplied to rice fields should be no greater than 2 mg/l.
In light of the foregoing, it can be seen that condensate produced from boron-containing geothermal steam poses a difficulty for geothermal plant operations. The boron originally present in the geothermal steam and/or in the steam flashed from the brine, ultimately, after passage through the turbine, becomes a component of the liquid steam condensate. The boron concentration in the steam condensate is oftentimes far in excess of the 2 mg/l limit, requiring as a practical consequence that neither water directly taken from the condenser nor from cooling tower blowdown be used for high value agricultural purposes or discharged into rivers, streams, and the like. Both the condensate and the cooling tower blowdown must eventually be re-injected into the earth for environmental protection.
Another problem with boron relates to cooling tower xe2x80x9cdriftxe2x80x9dxe2x80x94i.e., the moisture carried from the cooling tower into the air. If the moisture emitted from the cooling tower contains boron, say in a concentration of 2 mg/l or more, its ultimate deposition upon the ground can cause plant distress or death in the immediate vicinityxe2x80x94depending on the sensitivity of the local plants to boron. This xe2x80x9cdriftxe2x80x9d problem is especially acute with respect to geothermal power plants processing superheated steam taken directly from the geothermal formation. As such formations become depleted, the boron concentration in the steam produced increases, resulting in increased boron concentration in the drift. In particularly acute situations the boron concentration in the drift can be exceedingly highxe2x80x94on the order of 100-250 mg/lxe2x80x94due to a combination of high boron in the steam condensate make-up and a cooling tower run with high cycles of concentration.
Besides boron, another contaminant in geothermal steam which can accumulate in the condensate in undesirable concentrations for irrigation purposes or for discharge to inland waters or to the environment in general (e.g., by distribution upon the soil) is arsenic. Arsenic generally does not present as pervasive a problem for geothermal operations as boron since its concentration in geothermal steam is usually low, as is its concentration in the resulting steam condensate. Nevertheless, there are instances where the geothermal steam can contain arsenic in unusually large concentrations to produce a condensate containing arsenic in a concentration too high for discharge per local regulations. Generally, a limit of 0.1 mg/l will pertain for discharge to the environment, and 0.05 mg/l is the usual maximum for drinking water, although Pontius reports that values in the range of 0.002 to 0.020 mg/l for drinking water were under consideration in the U.S. in 1995. Plant toxicity to arsenic varies widely, van der Leeden et al. indicating that the tolerance varies from as much as 12 mg/l for Sudan grass to less than 0.05 mg/l for rice. Perhaps because rice cultivation is an important agricultural activity in the Philippines, the maximum value permitted for irrigation is 0.01 mg/l, per the Philippines NPCC Rules and Regulations of 1978 set forth hereinbefore. These same regulations, however, set a maximum limit of 0.05 mg/l for most other fresh surface waters, including water used as the source of public water supply.
Another contaminant in geothermal steam which on occasion can be found to accumulate in unacceptable concentrations in the steam condensate produced in the condenser is mercury. Mercury presents similar difficulties in processing as discussed above with respect to boron, except that both mercury and arsenic are seldom present in sufficient concentration in the geothermal steam to cause a xe2x80x9cdriftxe2x80x9d problem with respect to the surrounding neighborhood of the cooling tower.
It has been discovered in this invention that, as water condenses from steam in a condensing zone, as for example during passage through a turbine/condenser system in a geothermal power plant, contaminants such as boron, arsenic, and mercury are preferentially removed from the steam phase with the first liquid condensed. Hence, in the invention, a substantial proportion of one or more of such contaminants is captured in a contaminants-rich condensate produced at an early stage of steam condensation in the condensing zone while separately producing a contaminants-lean condensate from a later stage of condensation.
The present invention provides a method especially useful in a geothermal power plant processing steam containing boron, arsenic, and/or mercury through a turbine-condenser system. In particular, the invention provides a method for substantially reducing the concentration of these contaminants in the steam condensate in the condenser of a geothermal power plant processing boron, arsenic, and/or mercury-contaminated steam by capturing a substantial proportion of one or more of these contaminants in a contaminant-rich fraction of steam condensate produced at an early stage of condensation in the turbine-condenser system. Preferably, enough of the contaminants are collected in this early produced fraction of steam condensate such that a later-produced fraction is contaminant-lean, containing the contaminants in individual concentrations suitable for beneficial use in agriculture or for discharge to a naturally occurring water body.
One specific method contemplated in the invention for removing these contaminants from a turbine-condenser system is by directing steam containing moisture produced in the turbine via the turbine drains to a steam/water separator operating subatmospherically. The moisture, rich in the contaminants but oxygen free, is collected either as a liquid for disposal or for such purposes as water sealing while the separated steam, lean in contaminants, is directed to the condenser.
Another specific method involves (1) extracting one or more streams of steam/moisture mixtures from an interstage location in the turbine, (2) demisting the steam to produce an oxygen-free, contaminant-rich water and a demisted steam, and then (3) directing the demisted steam to an ejector drawing non-condensable gases from the main condenser for condensing steam from the turbine exhaust.
In addition, the invention provides for enhancing the removal of boron, arsenic, mercury, and other contaminants from geothermal steam by scrubbing the steam prior to entry into the turbine using an aqueous alkaline liquid absorbent.
Further still, the invention provides a method for collecting moisture in the turbine exhaust between the turbine and the condenser, which for purposes herein includes the general area of the entryway into the condenser. The collected moisture, being oxygen-free, is then a useful liquid for use in many locations in the geothermal power plant.
The invention also provides a method, particularly applicable for lowering maintenance costs in operating a cooling tower associated with a power facility introducing an ammonia- and/or hydrogen sulfide-containing steam through a turbine-condenser system, wherein at least some of the ammonia or hydrogen sulfide is captured in an early produced fraction of the total condensate produced in said turbine-condenser system while a later produced fraction of lower ammonia or hydrogen sulfide content is used as make-up to the cooling tower. Since both ammonia and hydrogen sulfide are nutrients for microorganisms, lowering the rate at which such nutrients are introduced into the cooling tower system slows the growth rate of such microorganisms, which in turn lowers the rate at which biocide (and its attendant costs) must be fed to the cooling tower system to reduce fouling thereof. Maintenance costs are also reduced resulting from the longer time spans between outages necessitated for cleaning of the cooling tower system.